Purpose: This study aimed to systematically review and analyze the effects of online mindfulness-based interventions (MBIs) on symptoms and health-related quality of life (HRQoL) in adults. The focus was on symptoms such as anxiety, depression, fatigue, sleep disturbances, and pain. Methods: A comprehensive literature search was conducted across seven electronic databases to identify relevant studies. Covidence was utilized to manage the review process, and the Joanna Briggs Institute Critical Appraisal Checklist was employed to assess the quality of each study. Results: A total of 31 studies met the predefined inclusion criteria. The analysis revealed significant positive effects of online MBIs on anxiety and depression across various delivery modes, including websites and mobile applications. Notably, studies utilizing websites consistently demonstrated high effectiveness, whereas mobile applications exhibited inconsistent findings, particularly with shorter intervention durations (less than 4 weeks) being less effective. Regarding HRQoL, six studies were analyzed, with four exhibiting significant improvements; all involved 8-week adapted MBI programs. Conclusion: This review systematically presents the current state of research on online MBIs for adults with symptoms and comprehensively analyzes their effects, providing directions for future program development. The findings highlight the importance of structured and sustained interventions to achieve meaningful improvements in symptoms and HRQoL.